This invention relates to lenses of configuration for light scanning through the lens, such as f-theta lens, the lenses being of material which changes or distorts the optical function with increases in humidity. This invention renders such lenses resistant to such changes or distortions.
Materials forming lenses that change or distort with increases of humidity are typically plastics, especially acrylic plastics. Some hydrocarbon plastics resist water intake and are used in special situations for lenses, but the material are expensive and are difficult to mold to form the lens.
The prior art has addressed this problem in U.S. Pat. No. 5,409,360 to Nakanishi, but the solution presented in that patent is one which significantly restricts the characteristics and form of the lens since it is based on the premise that a lens fully permeated with water does not exhibit major distortion. Accordingly, the better lenses in accordance with this patent would be tall and thin.
Lenses of this invention may differ little from ordinary form for scan lenses, but have vapor barriers applied to render the lens highly resistant to increases in humidity.
A lens of this invention has a long dimension, an optical transmission dimension perpendicular to the long dimension, and a height dimension perpendicular to both the long dimension and the optical transmission dimension. The lens has a clear aperture along the long dimension of at lease 35 millimeters (mm), the clear aperture having a height that is less than 32 mm. The clear aperture is surrounded top and bottom by lens body having height on each side of at least 1.5 mm.
On the top and bottom of this lens is a layer of a vapor barrier, which may be a sheet of metal or other imperious material attached by adhesive. Such lenses are very stable optically with changes in humidity up to almost saturation humidity. Where a lens is thin in the light transmitting direction, the barrier might be omitted at such thin points, as the significant optical distortion does not occur where the lens is thin enough to become fully saturated.